1. Field of the Invention
The invention relates to a device for black and white level stabilization, and to a display apparatus comprising such a device.
2. Description of the Related Art
In a CRT, the relation between the beam current and the light output is linear, so that an indirect information of the light intensity can be obtained by measuring the current values of the three cathodes for the R, G and B signals. The value of the current for each beam normally ranges from black to peak white. The black level is set by adjusting the cut-off values with the default (minimum) brightness level and without any video signal input. This current level is near to the zero emission (raster). The white level is set by adjusting the gain values of the three RGB channels with the default (maximum) contrast level and the maximum video signal input, in order to get the correct white temperature and light output. Both the black and the white levels are obtained by setting the cathode voltages.
During the lifetime of the CRT, the cathode efficiency decreases, so the settings of the tube are not correct anymore. The voltage/current characteristic of the cathodes changes causing a dark level shift and an error in the color temperature and in white level. In addition, a temperature drift causes errors in light emission.
The target of the Black and White Level Stabilization (BWLS) feature is to restore the original values of the current of the three beams by re-adjusting the cut-off and white voltages, in order to correct the light emission and the color deviations. To this purpose, the beam current values are measured at the end of the factory adjustments for the cut-off and white for each color temperature, then these reference values are stored in the monitor EEprom. When the BWLS adjustment routine is started, the actual beam current values are compared with the reference values and, if different, actions are taken to restore the original settings. Since the current measurement, especially for the white, must be carried out during the video active signal the BWLS routine must be recalled by means of a special command when the user decides to do it. This command can be an item of the On-Screen Display (OSD) menu. In order to realize the black and white level stabilization, a black/white pattern generator is needed inside the monitor.
It is, inter alia, an object of the invention to provide a very simple black and white level stabilization. To this end, a first aspect of the invention provides a device as for black and white stabilization. A second aspect of the invention provides a display apparatus incorporating such a black and white stabilization device.
In a black and white level stabilization device in accordance with a primary aspect of the invention, an OSD signal generator generates black and white reference signals, amplifying circuits amplify the black and white reference signals or RGB signals, and a measuring circuit, coupled to the amplifying circuits, measures a white level and a black level in response to the black and white reference signals.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.